Ni-based alloys have been used as weld metals for a pressure vessel and a steam generator in a light-water reactor for nuclear power generation. Overlay welding with Ni-based alloys have involved using Ni-15Cr-based or Ni-20Cr-based wire. As a measure against primary water stress corrosion cracking (PWSCC) generated in weld metal of Ni-15Cr-based or Ni-20Cr-based wire in pure water, which is primary cooling water, Ni-30Cr-based wire becomes more widely used. A weld metal of Ni-30Cr-based wire, however, tends to cause hot cracking at high temperature during welding compared with that of Ni-15Cr-based or Ni-20Cr-based wire.
The types of hot cracking include solidification cracking in which a liquid phase remaining at the final solidification position before complete solidification of molten weld metal forms openings due to distortion caused by solidification shrinkage and thermal shrinkage; liquation cracking in which impurity element-rich crystal grain boundaries heated to high temperature with welding heat in a subsequent pass in multilayer welding liquefy and form openings; and ductility-dip cracking in which grain boundaries having low cohesive strength in the middle-temperature range not higher than the solidus temperature form openings in response to stress acting on the grain boundaries.
A weld metal of Ni-30Cr-based wire has lower tensile strength than a weld metal of Ni-15Cr-based or Ni-20Cr-based wire. As a result, the weld metal of Ni-30Cr-based wire has lower tensile strength than a base metal, and Ni-30Cr-based wire thus imposes a restriction in device design.
To solve the above-described issues, PTL 1 discloses a Ni-based alloy solid wire for welding having a composition that contains specific amounts of Cr, Ti, Nb, Ta, C, and Fe and in which Al, N, Zr, Mg, P, S, Si, and Mn are controlled to specific amounts or less, with the balance being Ni and inevitable impurities. PTL 1 describes that the Ni-based alloy solid wire for welding provides weld metal with the resistance to ductility-dip reheat cracking, improves the tensile strength of weld metal to larger or equal to that of a base metal, and offers good welding workability.
PTL 2 discloses a high Cr content Ni-based alloy welding material having a composition that contains specific amounts of C, Si, Mn, Cr, Mo, Fe, Cu, Nb+Ta, Al, and Ti and further contains specific amounts of P and S as inevitable impurities, with the balance being Ni. PTL 2 describes that the high Cr content Ni-based alloy welding material can increase tensile strength.
PTL 3 discloses a wire-shaped product formed of a Ni—Cr—Fe alloy containing specific amounts of Cr, Fe, Mn, Nb+Ta, Mo, Si, Ti, Al, Cu, Zr, S, B, C, P, and Mg+Ca, with the balance being Ni and inevitable impurities. PTL 3 describes that the wire-shaped product can provide desired strength and desired resistance in addition to the resistance to solidification cracking, ductility-dip cracking, root cracking, and stress corrosion cracking.
PTL 4 discloses a Ni-based high Cr alloy weld wire having a composition that contains specific amounts of C, Mn, Fe, Si, Al, Ti, Cr, Ta, and Mo and further contains specific amounts of Ca+Mg, N, P, O, S, H, Cu, and Co as inevitable impurities, with the balance being Ni. PTL 4 describes that the Ni-based high Cr alloy weld wire can improve the tensile strength of weld zones, the resistance to weld cracking, the quality of the microstructure of weld metal, and the welding workability.